ABSTRACT Stem cells, including mesenchymal stem cells (MSCs) have proven to be an exciting and promising thera- peutic for the treatment of numerous diseases and disorders of the spinal cord, such as spinal cord injury (SCI) and amyotrophic lateral sclerosis (ALS). Positive results in preclinical studies have led to an increase in transla- tion of stem cell therapies. However, upon reaching clinical trials, many stem cell therapeutics fail. In search for answers, the clinicians and researchers are missing critical information; the fate of the cells once implanted. Therefore, there is a definite and urgent clinical need for a technique that is capable of accurate real-time guid- ance of quantitative stem cell delivery to target areas, noninvasive longitudinal in vivo tracking of stem cell loca- tion, and assessment of stem cell viability over time after the delivery. This project is based on a hypothesis that stem cells can be effectively labeled with a photoacoustic imaging contrast agent and a photoacoustic apoptosis sensor to visualize cell localization and ascertain their viability, respectively; and ultrasound-guided photoacous- tic (USPA) imaging system and approach can be developed for longitudinal, quantitative, and noninvasive track- ing of double-labeled stem cells and their fate in vivo. The overall goal of this research program is to develop a novel cell labeling system and corresponding USPA imaging approach allowing real time image guidance for precise and accurate injection of the stem cells, and longitudinal tracking both the location and viability of stem cells in vivo in the spinal cord. First, a PA sensitive tracker will be developed, which will allow for real time feedback on the location of the stem cells during and immediately after injection, as well as post-operative tracking the location of the transplanted cells over time. Second, a PA sensor of apoptosis will be developed. This sensor will provide a unique PA signal in cells which are undergoing apoptosis, allowing us to ascertain the cells viability. The combination of both PA tracker and apoptotic sensor to double label stem cells is critical for real time imaged guided delivery of cells, and tracking viable and apoptotic cells longitudinally in vivo. The USPA imaging double-labeled cells will provide highly valu- able information of the fate of stem cells in vivo, which can be used to refine and advance the field of stem cell transplantation in the spinal cord. If successful, this work will lay foundation for in vivo real-time intra-operative and then longitudinal USPA imaging of transplanted cells within the spinal cord ? the technology needed by both scientists and clinicians.